1. Field of the Invention
The present invention relates to a cold accumulating material, a method of manufacturing the material and a cold accumulating type refrigerator using the cold accumulating material, and more particularly to a cold accumulating material which is free from the risk of being pulverized into fine particles, and is excellent in mechanical strength and durability, and capable of exhibiting a significant refrigerating performance at a low temperature region, and relates to a method of manufacturing the cold accumulating material, refrigerator using the cold accumulating material and various application devices using the regenerator.
2. Description of the Related Art
Recently, superconductivity technology has been progressed remarkably and with an expanding application field thereof, development of a small, high performance refrigerator has become indispensable. For such a refrigerator, basic requirements such as light weight, small size and high heat efficiency are demanded, and a small-sized refrigerator has been practically applied to various industrial fields.
For example in a super-conductive MRI apparatus, cryopump and the like, a refrigerator based on such refrigerating cycle as Gifford MacMahon type (GM refrigerator), Starling method, pulse-tube type refrigerator has been used. Further, a magnetic floating (levitating) train absolutely needs a high performance refrigerator for generating magnetic force by using a super-conductive magnet. Further, in recent years, a super-conductive power storage apparatus (SMES) or an in-magnetic-field single crystal pull-up apparatus (magnetic field applied Czochralski) has been provided with a high performance refrigerator as a main component thereof. Further, a development and practical use of the pulse tube type refrigerator has been aggressively advanced, because the pulse tube type refrigerator is expected to attain a high reliability.
In the above described refrigerator, the operating medium such as compressed He gas or the like flows in a specified direction in a regenerator (cold accumulating unit) filled with cold accumulating materials so that the heat energy thereof is supplied to the cold accumulating material. Then, the operating medium expanded here flows in an opposite direction and receives heat energy from the cold accumulating material. As the recuperation effect is improved in this process, the heat efficiency in the operating medium cycle is improved so that a further lower temperature can be realized.
As a cold accumulating material for use in the above-described refrigerator, conventionally Cu, Pb and the like have been used. However, these cold accumulating materials have a very small specific heat in extremely low temperatures below 20 K. Therefore, the aforementioned recuperation effect is not exerted sufficiently, so that even if the refrigerator is cyclically operated under an extremely low temperature, the cold accumulating material cannot accumulate sufficient heat energy, and it becomes impossible for the operating medium to receive the sufficient heat energy. As a result, there is posed a problem of that the refrigerator in which the regenerator (cold accumulating unit) filled with aforementioned cold accumulating material is assembled cannot realize the extremely low temperatures.
For the reason, recently to improve the recuperation effect of the regenerator at extremely low temperature and to realize temperatures nearer absolute zero, use of magnetic cold accumulating material made of intermetallic compound formed from a rare earth element and transition metal element such as Er3Ni, ErNi, ErNi2, HoCu2 having a local maximum value of volumetric specific heat and indicating a large volumetric specific heat in an extremely low temperature range of 20 K or less has been considered. By applying the magnetic cold accumulating material to the GM refrigerator, a refrigerating operation to produce an arrival lowest temperature of 4 K is realized.
With the advance of reviews for practically applying the above refrigerator into various refrigerating systems, a technical demand for cooling and refrigerating a large-scaled object under a stable state for a long time has been increased, so that it is required to further improve the refrigerating performance (refrigerating capacity) of the refrigerator.
In order to cope with the above technical demand, there has been tried a countermeasure in which a part of the cold accumulating material composed of conventional metal-type magnetic particles generally used is substituted with an oxide-type magnetic cold accumulating material such as GdAlO3 or the like containing rare earth element, so that a specific heat characteristic of the whole cold accumulating material are suitably controlled thereby to improve the refrigerating capacity.
The magnetic cold accumulating material described above is normally worked and used in a form of spherical-shape having a diameter of about 0.1-0.5 mm for the purpose of smoothly flowing He gas, and for effectively performing the heat exchange with He gas as cooling medium in the refrigerator thereby to stably maintain the heat exchange efficiency. In particular, in a case where the magnetic cold accumulating material (particulate cold accumulating substance) is intermetallic compound containing rare earth element, the particulate cold accumulating substance is worked so as to provide a spherical-shape in accordance with working methods such as centrifugal atomizing method.
However, in the above oxide-type magnetic cold accumulating material, since the oxide substance has a high melting point, it is impossible to work the oxide substance so as to provide spherical shape in accordance with a centrifugal spraying method through which the conventional metal-type magnetic cold accumulating material has been worked. Therefore, the conventional oxide-type magnetic cold accumulating material has been worked and manufactured so as to provide a shape close to sphere in accordance with a method comprising the steps of granulating a fine raw material powder to form granulated particles each having an appropriate size; and sintering the granulated particles.
Further, in a Starling-type refrigerator and a pulse-tube type refrigerator or the like to be operated with a high speed, there has been posed a problem that a pressure loss at the regenerator packed with spherical magnetic cold accumulating particles is disadvantageously increased, so that a sufficient refrigerating capacity cannot be realized. Further, in the GM refrigerator or the like, there has been liable to cause the following disadvantages. Namely, pressure vibrations caused by a highly pressurized He gas, various stresses and impact forces are applied to the magnetic body particles (magnetic cold accumulating particles) during the operation of the refrigerator and the magnetic particles were liable to be further finely pulverized, so that a flow resistance of the cooling medium gas is increased thereby to abruptly lower the heat exchange efficiency.
In particular, in case of the GM refrigerator, a stress caused by reciprocal movement of a displacer (i.e. a piston for compressing the cooling medium) is applied to the cold accumulating material, thus exerting a great influence on the characteristic of the cold accumulating material. Further, at a time of starting the refrigerator, the temperature of the regenerators of the refrigerator rapidly lowered in a short time from a room temperature (RT) to an extremely low temperature close to about 4 K, so that a large thermal shock (heat impact) is applied to the cold accumulating material.
However, in general, the oxide substances exhibit an extreme brittleness, insufficient mechanical strength and a small heat impact resistance, so that the following disadvantageous phenomena are liable to cause. That is, the oxide type cold accumulating material is broken during the operation of the refrigerator, a part of surfaces of the cold accumulating material peels off thereby to generate fine powders. The generated fine powders are liable to damage the seal portions of the refrigerator. As a result, there is posed a problem of remarkably lowering the refrigerating capacity of the refrigerator.
Therefore, in order to improve the mechanical strength of the oxide type cold accumulating material, there has been tried to take a measure so that a crystal structure of the cold accumulating material particles is made fine. However, when the crystal structure becomes fine, the crystal boundaries having a large heat resistance are disadvantageously increased thereby to deteriorate the thermal conductivity of the cold accumulating material. Then, when the thermal conductivity is lowered, the heat exchange between the cold accumulating material and He gas as the cooling medium gas becomes insufficient, so that the cold accumulating function cannot be sufficiently exhibited to a deep inner portion of the cold accumulating material particle, thereby to disadvantageously lower the refrigerating capacity.
Further, particularly in the oxide-type cold accumulating material manufactured by afore-mentioned method comprising the steps of granulating the fine oxide material powder to form granulated particles and then sintering the granulated particles, all of the material components are not molten or dissolved to each other, so that it is difficult to manufacture a completely densified cold accumulating particle. That is, there is manufactured particles formed with fine cracks on the surface of the particles, particles having a coarse surfaces formed by irregularities on the surface of the particles, particles formed with fine pores or voids in the inner portions of the particles. Therefore, due to the pressure vibrations and various stresses applied to the particles during the operation of the refrigerator, a breakage or finely pulverization of the particles are liable to cause from the defectives such as crack, irregularities, pore, void or the like. The generated fine powders are liable to damage the seal portions of the refrigerator. As a result, there is posed a problem of remarkably lowering the refrigerating capacity of the refrigerator.
The present invention has been achieved to solve the above described problems and an object of the invention is to provide a cold accumulating material which is free from the fear of being finely pulverized, and is excellent in thermal shock resistance and durability, and capable of exhibiting a significant refrigerating performance at an extremely low temperature range for a long period of time in a stable condition, and provide a cold accumulation refrigerator using the same.
In addition, another object of the present invention is to provide an MRI apparatus, a super-conducting magnet for magnetic floating train, a cryopump and an in-magnetic field single crystal pull-up apparatus capable of exerting an excellent performance for a long period of time by using the aforementioned cold accumulation refrigerator.
To achieve the above objects, the cold accumulating material of the present invention comprises a number of magnetic particles mainly composed of oxide wherein an average value of equivalent circle diameters of crystal grains constituting the magnetic particles is 0.3-20 xcexcm.
In the above cold accumulating material, it is preferable that an area ratio of the crystal grains each having an equivalent circle diameter of 50 xcexcm or more is 10% or less with respect to whole crystal grains constituting the magnetic particle.
Further, in the above cold accumulating material, it is preferable that the magnetic particles are composed of sintered bodies of granulated grains and sintering density of the sintered bodies is 86-99.8%. It is also preferable that the magnetic particles contain at least one element selected from the group consisting of Y, Mg, Al, Ca and rare earth elements in a range of 0.5-15 wt % calculated as oxide thereof, the selected elements being different from elements constituting the magnetic particles.
Furthermore, in the above cold accumulating material, it is also preferable that a ratio of the magnetic particles each of which surface is formed with at least two cracks each having a length of 10 xcexcm or more is 20% or less with respect to whole number of the magnetic particles.
Still further, in the above cold accumulating material, it is also preferable that a ratio of the magnetic particles each of which surface roughness in terms of maximum height (Rmax) is 10 xcexcm or more is 30% or less with respect to whole number of the magnetic particles.
Further, in the above cold accumulating material, it is also preferable that a ratio of the magnetic particle in which void or pore having a maximum width of 20 xcexcm or more exist is 40% or less with respect to whole number of the magnetic particles.
Furthermore, in the above cold accumulating material, it is also preferable to control so that the magnetic particles contain 3 ppm-2 wt % of silicon, sodium and iron in total amount thereof.
Furthermore, in the above cold accumulating material, it is also preferable that the magnetic particles are composed of oxide magnetic particles expressed by a general formula of Gd1xe2x88x92xRxA1xe2x88x92yByO3 wherein R denotes at least one of rare earth element selected from the group consisting of Ce, Pr, Nd, Pm, Sm, Tb, Dy, Ho and Er, while A denotes at least one element selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Al and Si, at least two elements being selected as A component in a case of x=0 and y=0, while at least one element being selected as A component in a case of xxe2x89xa00 or yxe2x89xa00, B denotes at least one element selected from the group consisting of Zr, Nb, Mo, Ag, In, Sn, Sb, Hf, Ta, W, Au, and Bi, and x in atomic ratio satisfies a relation: 0xe2x89xa6xxe2x89xa60.4, while y in atomic ratio satisfies a relation: 0xe2x89xa6xxe2x89xa60.4.
Furthermore, in the above cold accumulating material, it is also preferable that the magnetic particles are composed of oxide magnetic particles having at least one property selected from: a property exhibiting a specific heat of 0.3 J/K cm3 or more in a temperature range of 4.0-5.0 K; a property exhibiting a specific heat of 0.35 J/K cm3 or more in a temperature range of 4.5-5.5 K; and a property exhibiting a specific heat of 0.4 J/K cm3 or more in a temperature range of 5.5-6.0 K.
A method of manufacturing the cold accumulating material of the present invention comprises the steps of: granulating oxide powder to form granulated particles; press-treating thus obtained granulated particles to prepare densified particles each having a spherical shape; and conducting a sintering treatment to the densified particles thereby to prepare a cold accumulating material composed of a number of magnetic particles.
In the above method in accordance with the granulation method, it is preferable that the press-treatment for the granulated particles is a cold isostatic pressing (CIP) treatment. It is also preferable that the sintering treatment for the densified particles is a hot isostatic pressing (CIP) treatment. Further, it is also preferable that the method further comprises the steps of: adding 5-30 wt % of binder to the oxide powder to form a mixed powder; and granulating the mixed powder to form the granulated particles.
Another method of manufacturing cold accumulating material of the present invention comprising the steps of: melting oxide powder by being passed through heat-plasma to form a molten liquid; and solidifying the molten liquid in a state where the molten liquid is spheroidized by the action of surface tension of the molten liquid thereby to prepare a cold accumulating material composed of a number of magnetic particles.
In the method of manufacturing cold accumulating material in accordance with heat plasma method, it is also preferable that the method further comprises the step of conducting a heat treatment at a temperature of 500xc2x0 C. or more with respect to the magnetic particles spherically formed by being passed through the heat plasma. Further, it is also preferable that the temperature for the heat treatment is 1200-1700xc2x0 C.
A cold accumulation refrigerator of the present invention comprises a regenerator filled with a cold accumulating material through which a cooling medium gas flows from a high temperature upstream side of the regenerator, so that heat is exchanged between the cooling medium gas and the cold accumulating material packed in the regenerator thereby to obtain a lower temperature at a downstream side of the regenerator, wherein at least part of the cold accumulating material packed in the regenerator is composed of the cold accumulating material of the present invention.
In addition, when the high temperature side of the regenerator is packed with a conventional non-oxide type cold accumulating material, while the low temperature side of the regenerator is packed with the cold accumulating material of the present invention, it becomes possible to suitably control a specific heat distribution in the regenerator. As the afore-mentioned non-oxide type cold accumulating material, the materials are not particularly limited. The conventional materials such as Pb, HoCu2, Er3Ni or the like can be used.
Each of the MRI (Magnetic Resonance Imaging) apparatus, super-conducting magnet for the magnetic floating train, cryopump and in-magnetic field single crystal pull-up apparatus (magnetic field applied Czochralski) according to the present invention is characterized by comprising the cold accumulation refrigerator as described above.
The cold accumulating material of the present invention consists of a number of magnetic particles mainly composed of oxide having a peak of specific heat at an extremely low temperature region below 20 k. As the oxide constituting the magnetic particles, for example, compositions expressed by the following general formulae (1), (2), (3) and (4) are suitably used.
That is, there can be used a perovskite-type oxide expressed by a general formula:
RMO3xe2x80x83xe2x80x83(1)
wherein R denotes at least one of rare earth element selected from the group consisting of Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb, while M denotes at least one element selected from 3 B family elements.
There can be also used a spinel-type oxide expressed by a general formula:
AB2O4xe2x80x83xe2x80x83(2)
wherein A denotes at least one of rare earth element selected from 2 B family elements, while B denotes transition metal elements containing at least of Cr.
There can be also used an oxide expressed by a general formula:
CD2O6xe2x80x83xe2x80x83(3)
wherein C denotes at least one element selected from Mn and Ni, while D denotes at least one element selected from Nb and Ta.
Among the above oxides, GdAlO3 had been considered to be preferable, because this oxide had an extremely sharp and high specific heat peak at a low temperature region about 3.9 K. However, this oxide had a disadvantage of having a small specific heat at a high temperature side of 4 k or more. Therefore, in spite of having such a high peak of the specific heat, an improvement of the refrigerating capacity at 4.2 K was not sufficient.
Therefore, the present invention proposes a cold accumulating material having a composition expressed by the following general formula (4) as the cold accumulating material having a high specific heat peak at a high temperature side in comparison with that of the conventional cold accumulating material having a composition of GdAlO3.
Namely, it is preferable to use a cold accumulating material composed of an oxide magnetic particles expressed by a general formula of:
Gd1xe2x88x92xRxA1xe2x88x92yByO3xe2x80x83xe2x80x83(4)
wherein R denotes at least one of rare earth element selected from the group consisting of Ce, Pr, Nd, Pm, Sm, Tb, Dy, Ho and Er, while A denotes at least one element selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Al and Si, at least two elements being selected as A component in a case of x=0 and y=0, while at least one element being selected as A component in a case of xxe2x89xa00 or yxe2x89xa00, B denotes at least one element selected from the group consisting of Zr, Nb, Mo, Ag, In, Sn, Sb, Hf, Ta, W, Au, and Bi, and x in atomic ratio satisfies a relation: 0xe2x89xa6xxe2x89xa60.4, while y in atomic ratio satisfies a relation: 0xe2x89xa6xxe2x89xa60.4.
In the cold accumulating material of the present invention, it is preferable that the magnetic particles are composed of oxide magnetic particles having at least one property selected from: a property exhibiting a specific heat of 0.3 J/K cm3 or more in a temperature range of 4.0-5.0 K; a property exhibiting a specific heat of 0.35 J/K cm3 or more in a temperature range of 4.5-5.5 K; and a property exhibiting a specific heat of 0.4 J/K cm3 or more in a temperature range of 5.5-6.0 K.
When the cold accumulating materials having various specific heat characteristics were packed in the refrigerator and refrigerating tests were conducted, the inventors of the present invention had obtained the following knowledge. That is, in order to improve the refrigerating capacity at 4 K, it was confirmed that at least one of the aforementioned characteristics of the specific heat at the three temperature regions should be satisfied. Among the above three specific heat characteristics, it is preferable to satisfy two characteristics of the specific heat. Further, it is more preferable to satisfy all of the three characteristics of the specific heat.
Regarding the general formula (4) of Gd1xe2x88x92xRxA1xe2x88x92yByO3, in a case of x=0 and y=0, the general formula (4) can be expressed by a formula of GdAO3. In this oxide composition of GdAO3, however, when the A component is composed of a single element, there can be generally obtained a magnetic body having a specific heat at an extremely low temperature region, while the magnetic body rarely exhibits a high specific heat at the extremely low temperature range of 4-6 K. Therefore, in a case of x=0 and y=0, at least two elements are selected as A component. On the other hand, when a part of Gd is substituted for the other rare earth element, or when a part of A component is substituted for the other element, it becomes possible to control the specific heat characteristics of the magnetic body thereby to obtain a cold accumulating material having an excellent performance.
In the above general formula (4) of a general formula of Gd1xe2x88x92xRxA1xe2x88x92yByO3, R component denotes at least one of rare earth element selected from the group consisting of Ce, Pr, Nd, Pm, Sm, Tb, Dy, Ho and Er, and R is an effective component for broadening a sharpened specific heat peak and controlling the position of the peak temperature. The R component is added so as to substitute a part of Gd. When the addition ratio x indicating the substituting amount of R component exceeds 0.4, the specific heat of the magnetic body is disadvantageously lowered. Among the above R component, Tb, Dy, Ho and Er are preferable, and Tb and Dy are more preferable.
Further, A component denotes at least one element selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Al and Si, and has an effect of controlling the peak of specific heat. At least two elements are selected in a case of x=0 and y=0, while at least one element is selected in a case of xxe2x89xa00 or yxe2x89xa00, so that a part of Gd or A component in GdAO3 type magnetic body is invariably substituted for the other element. Among the above a component elements, of Ti, V, Cr, Mn, Fe, Co, Ni, Ga and Al are preferable, and Cr, Mn, Fe, Co, Ni, Ga and Al are more preferable.
Furthermore, B component is an element for improving the specific heat characteristic by the function of controlling a distance between atoms of (Gd1xe2x88x92xRx) when B component is substituted for a part of A component. The B component denotes at least one element selected from the group consisting of Zr, Nb, Mo, Ag, In, Sn, Sb, Hf, Ta, W, Au and Bi. As the B component element, Zr, Nb, Mo, Sn, Ta and W are preferable, and Ta and W are more preferable. When the addition ratio y indicating the addition amount of B component exceeds 0.4, it becomes impossible to maintain the perovskite structure, so that the specific heat characteristics of the cold accumulating material composed of the magnetic body is disadvantageously lowered.
Furthermore, there may be a case where the atomic ratio of oxygen in the above general formula: Gd1xe2x88x92xRxA1xe2x88x92yByO3 is deviated from a stoichiometric ratio of 3 due to atomic defectives or the like. However, if the atomic ratio of oxygen is within a range of 2.5-3.5, the above deviation has not a great influence on the specific heat characteristic of the magnetic body.
An average value of the equivalent circle diameters of the crystal grains of the magnetic particles constituting the cold accumulating material of the present invention is set to 0.3-20 xcexcm. In this connection, as shown in FIG. 16, the equivalent circle diameter D of the crystal grain 2 is defined as a diameter D of a normal circle having a sectional area A which is equal to an exposed area or a sectional area of the crystal grain 2 of the magnetic particle when a surface structure or a sectional structure of the magnetic particle is observed. The average value is calculated by averaging the equivalent circle diameters of 100 crystal grains sampled arbitrarily.
By the way, since the cold accumulating material of the present invention is constituted by the magnetic particles mainly composed of an oxide, when the magnetic particle is lapped or polished in order to observe the sectional structure of the magnetic particle, grain boundary phases are collapsed by polishing agent, so that the boundaries of the crystal grains become unclear, whereby there may be a case where the measuring of the equivalent circle diameter of the crystal grain becomes difficult. Even in such a case, it is possible to measure the size of the crystal grain revealed and exposed on the surface structure of the magnetic particle.
When the average value of the equivalent circle diameters of the crystal grains of the magnetic particles is less than 0.3 xcexcm, the crystal grain boundaries having a high heat resistance are disadvantageously increased thereby to deteriorate a thermal conductivity of the magnetic particles, thus being not preferable. On the other hand, when the average value of the equivalent circle diameters of the crystal grains exceeds 20 xcexcm, a mechanical strength of the magnetic particle becomes insufficient. Accordingly, the average value of the equivalent circle diameters of the crystal grains is set to a range of 0.3-20 xcexcm. However, a range of 0.5-10 xcexcm is more preferable. Further, a range of 1-7 xcexcm is more preferable.
In the cold accumulating material of the present invention, it is also preferable that an area ratio of the crystal grains each having an equivalent circle diameter of 50 xcexcm or more is 10% or less with respect to whole number of crystal grains constituting the magnetic particle. In a case where the above area ratio exceeds 10%, a thermal shock caused by a rapid descent of temperature to be occurred at the time of starting the refrigerator is applied to the magnetic particles, so that cracks are formed to the magnetic particles whereby the particles are liable to be easily broken. These phenomena are assumed to cause due to the following reason. That is, it is considered that a rapid shrinkage of the crystal grain having a larger equivalent circle diameter exceeding 50 xcexcm cannot be fully absorbed and relieved by the whole crystal structure of the magnetic particle whereby the cracks are liable to occur.
The more preferable range of the area ratio of the crystal grains each having the equivalent circle diameter of 50 xcexcm or more is 5% or less. In addition, it is more preferable that the area ratio of the crystal grains each having the equivalent circle diameter of 40 xcexcm or more is 10% or less. Further, it is more preferable that the area ratio of the crystal grains each having the equivalent circle diameter of 30 xcexcm or more is 10% or less.
The size of the crystal grain can be adjusted by controlling various manufacturing conditions such as sintering temperature, sintering time, heating speed of the molded material body, cooling speed after the sintering operation, impurity content in the material molded body or the like. However, these manufacturing conditions are complicatedly affected to each other, and greatly influenced by factors inherent to a sintering furnace or the like, so that it is difficult to simply specify the manufacturing conditions.
However, in general, when the sintering temperature is set to high or the sintering time is prolonged, there is a tendency of increasing the size of the crystal grains. In the same manner, when both the heating speed at the sintering operation and the cooling speed after the sintering operation are lowered, there is a tendency of the crystal grains being grown to be coarsened. In addition, an impurity is one factor of generating nucleus of crystal, so that the size of the crystal grain is easily increased when the impurity content is small.
The measuring and evaluation of the crystal grain size can be performed through an image processing of a structural chart obtained by observing the surface structure or the sectional crystal structure of the magnetic particle by means of a scanning-type electron microscope (SEM).
In a case where the cold accumulating material of the present invention is constituted by the magnetic particles composed of sintered bodies of granulated grains, it is preferable that the sintering density (relative density) of the magnetic particles is set to a range of 86-99.8%. When the sintering density is less than 86%, the mechanical strength of the magnetic particle becomes insufficient and a packing amount of the magnetic particles into a regenerator is disadvantageously lowered, thus being not preferable. On the other hand, when the above sintering density exceeds 99.8%, the thermal shock caused by a rapid descent of temperature to be occurred at the time of starting the refrigerator is applied to the magnetic particles, so that the cracks are liable to occur, thus being not preferable. A more preferable range of the sintering density is set to a range of 95-99.8%. A more preferable sintering density is 98-99.8%. On the other hand, in a case where the cold accumulating material is constituted by the magnetic particles manufactured in accordance with a heat-plasma method, the density of the magnetic particles can attain to 99-100%.
It is preferable that the magnetic particles constituting the cold accumulating material contain at least one element selected from the group consisting of Y, Mg, Al, Ca and rare earth elements in a range of 0.5-15 wt % calculated as oxide thereof, and the selected elements are different from elements constituting the magnetic particles.
The oxide as a main phase constituting the magnetic particles has a peculiar peak of the specific heat at an extremely low temperature region below 40 K, and has a function as cold accumulating material. When at least one element selected from the group consisting of Y, Mg, Al, Ca and rare earth elements in a range of 0.5-15 wt % calculated as oxide thereof that are different from elements constituting the main phase are contained, the above oxide sintered body can be further densified. By densifying the respective magnetic particles, there can be realized the magnetic cold accumulating material composed of a composite oxide having a high mechanical strength and an excellent thermal shock resistance.
An adding component containing at least one element selected from the group consisting of Y, Mg, Al, Ca and rare earth elements that are different from the component constituting the main phase of the above magnetic particles is generally added in a form of oxide. However, the adding component is not limited to oxide, but can be also added as compounds such as carbide, nitride or the like. Among the above adding components, Y, Ce, Mg and Ca are particularly preferable for obtaining the densifying effect.
When the addition amount of the adding component is less than 0.5 wt % calculated as oxide thereof, the effect of densifying the sintered body is small. On the other hand, when the addition amount exceeds 15 wt %, a ratio of the main phase constituting the magnetic particle is relatively lowered thereby to deteriorate the cold accumulating effect. Therefore, the addition amount is set to 0.5-15 wt %, however, a preferable range of the addition amount calculated as oxide thereof is 1-10 wt %. A range of 2-7 wt % is more preferable.
Further, in the cold accumulating material of the present invention comprising a number of magnetic particles mainly composed of oxide, it is preferable that a ratio of the magnetic particles each of which surface is formed with at least two cracks each having a length of 10 xcexcm or more is 20% or less with respect to whole number of the magnetic particles.
When there exists a plurality of cracks on a surface of the magnetic particles constituting the cold accumulating material, the cracks are liable to advance by vibration and impact force applied thereto at the time of operation of the refrigerator, so that a possibility of the particles being destroyed is disadvantageously increased. To put it more concretely, when an existing ratio (number ratio) of the magnetic particles each of which surface is formed with at least two cracks each having a length of 10 xcexcm or more exceeds 20%, the ratio of the particles to be broken is increased. As a result, the generated fine powder will damage the seal portions of the refrigerator thereby to lower the performance of the refrigerator.
Therefore, the existing ratio of the magnetic particles each of which surface is formed with at least two cracks each having a length of 10 xcexcm or more is set to 20% or less. However, a ratio of 10% or less is more preferable, and a ratio of 5% is furthermore preferable. In this connection, it is more preferable that the crack to be an object to be measured is a crack having a length of 5 xcexcm or more. It is furthermore preferable that the crack to be an object to be measured is a crack having a length of 3 xcexcm or more.
Further, it is preferable that the cold accumulating material comprises a number of magnetic particles mainly composed of oxide and a ratio of the magnetic particles, each of which surface roughness in terms of maximum height (Rmax) is 10 xcexcm or more, is 30% or less with respect to whole number of the magnetic particles.
In a case where the surface roughness of the magnetic particle is large, a stress concentration is liable to occur at portions formed with projections or irregularities, so that the particle is broken at the stress concentrated portion as a starting point of the breakage. To prevent such a phenomenon, the ratio of the magnetic particles each having a maximum height of (Rmax) showing a degree of surface roughness is set to 30% or less. It is more preferable that the ratio of the magnetic particles each having a maxim height of 10 xcexcm or more is set to 30% or less. It is more preferable that the ratio of the magnetic particles each having a maximum height of 10 xcexcm or more is set to 20% or less. The ratio of 10% or less is furthermore preferable.
In this connection, it is more preferable that the maximum height of the surface roughness to be an object to be evaluated is a maximum height of 5 xcexcm or more. Furthermore, the maximum height of 3 xcexcm or more is furthermore preferable to be adopted as the object to be evaluated. The aforementioned surface roughness can be measured in accordance with Japanese Industrial Standard (JIS-B0601) in which the surface structure is photographed by means of an observing means such as a scanning-type electron microscope (SEM) or the like, then the surface roughness can be obtained from a cross-sectional curve of thus obtained photographic image.
Further, in the cold accumulating material of the present invention comprising a number of magnetic particles mainly composed of oxide, it is preferable that a ratio of the magnetic particles in which void or pore having a maximum width of 20 xcexcm or more exist is 40% or less with respect to whole number of the magnetic particles.
In this connection, the maximum width of the pore or void is measured as a shorter-side length of a quadrangle having a minimum area after enclosing a sectional shape of the pore or void exposed on a cross-sectional area of the magnetic particle.
In also a case where the pores or voids are formed within the magnetic particles, the mechanical strength of the magnetic particles is lowered, so that the magnetic particles are liable to be broken during the operation of the refrigerator.
Therefore, the ratio of the magnetic particles in which void or pore having a maximum width of 20 xcexcm or more exist is set to 40% or less. However, a ratio of 30% or less is more preferable, and a ratio of 20% is furthermore preferable. In this connection, it is more preferable that the width of the pore or void to be an object to be measured is a pore or void having a width of 5 xcexcm or more. The width of 3 xcexcm or more is furthermore preferable.
The method of measuring the ratio of the magnetic particles having defects such as aforementioned cracks, the maximum height of surface roughness, pore or void or the like is not particularly limited. However, for example, the ratio can be measured in accordance with the following method. That is, there can be adopted a method in which 20 or more of the magnetic particles are sampled at random from the cold accumulating material composed of a number of magnetic particles, then defective circumstances of cracks, maximum height, pore or void or the like are observed through the observing means such as electron microscope or the like, thereafter, the ratio of the magnetic particles having a defect is calculated. In this case, in order to measure the ratio of the particles having the defect with high accuracy, it is preferable that the number of the particles to be an object for the observation is set to 50 or more. Further, 100 or more of the particles are furthermore preferable.
In this regard, in case of the observation of the cracks, it is sufficient to observe only one side surface of the respective particles. Namely, when a group of the particles is observed at one visual field, it is not required to consider opposite side surfaces each forming a shadow of the respective particles. On the other hand, in case of measuring the surface roughness and the internal defects of the magnetic particles, the objective particles are buried in a base material such as resin or the like. Thereafter, a surface of the base material is lapped or polished so as to expose a cross-sectional area of the particle, then the cross-sectional area is observed by a microscope, thus being a suitable method of measuring the defectives. In this case, a cross-sectional area of a particle having a diameter corresponding to 80-120% of the average diameter of the magnetic particles shall be selected as an object for the measurements.
Further, in the cold accumulating material of the present invention comprising a number of magnetic particles mainly composed of oxide, it is also preferable that the magnetic particles contain 3 ppm-2 mass % of silicon, sodium and iron in total amount thereof.
The inventors of the present invention had paid attention to a fact that a precipitated substance (deposit) precipitated in the grain boundaries at a small amount has a great influence on the strength of the sintered body. As a result of the continuous researches, the inventors of this invention had obtained a knowledge of that the strength of the sintered body is disadvantageously lowered when a large amount of compounds such as oxides or the like of silicon (Si), sodium (Na) and iron (Fe) are precipitated in the grain boundaries. That is, the inventors had obtained a finding of that when a total amount of the Si, Na and Fe exceeds 2 mass %, the strength as the cold accumulating material is disadvantageously lowered.
On the other hand, when the total amount of the Si, Na and Fe is less than 3 ppm, the amount of the precipitated substance for suppressing the crystal growth is extremely lowered thereby to coarsen the crystal grains. When the crystal grains are grown and coarsened, the mechanical strength of the magnetic particles is disadvantageously lowered, and the thermal shock resisting property is also deteriorated.
Accordingly, it is preferable that the total amount of silicon, sodium and iron is set to 3 ppm-2 mass %. However, an amount of 10 ppm-1 mass % is more preferable, and an amount of 50-5000 ppm is furthermore preferable. In this connection, in the aforementioned general formula (4), when at least one element of Si and Fe is selected as A component, the aforementioned total amount means an amount excluding the amount contained as A component in general formula (4).
The method of manufacturing the cold accumulating material of the present invention is not particularly limited. However, for example, the cold accumulating material can be manufactured in accordance with a method comprising the steps of: mixing a material powder by means of a ball mill or the like to prepare a material mixture; spherically molding (granulating) thus obtained material mixture through rolling-granulation method, agitating granulation method, extrusion method, atomizing method (spraying method) or press-molding method or the like; and sintering thus obtained spherical molded (granulated) body.
The material powder to be used in the above manufacturing method shall preferably be a powder having a grain size of 0.3-30 xcexcm. A more preferable range of the grain size is 0.4-10 xcexcm. Further, a range of 0.5-8 xcexcm is furthermore preferable.
By the way, there may be a case where the molded particles or granulated particles manufactured through rolling-granulation method, agitating granulation method, extrusion method, atomizing method (spraying method) or press-molding method or the like has a low molding-density, so that there is a case where the molded (granulated) particles cannot be sintered particles having a good property even after a sufficient sintering operation.
Therefore, the present invention also adopts the following manufacturing method.
Namely, there can be adopted a method of manufacturing the cold accumulating material comprising the steps of: granulating oxide powder to form granulated particles; press-treating thus obtained granulated particles through a cold-isostatic pressing method to prepare densified particles each having a spherical shape; and conducting a sintering treatment to the densified particles thereby to prepare a cold accumulating material composed of a number of magnetic particles.
In the aforementioned manufacturing method, a hot isostatic pressing (HIP) treatment may be conducted as the press-treatment. By conducting the hot isostatic pressing (HIP) treatment or the cold isostatic pressing (CIP) treatment to the granulated particles, the density of the molded body can be further improved. Furthermore, when the molded body having such a high density is sintered, the magnetic particle having less cracks and pores or voids can be effectively obtained.
Further, in the aforementioned manufacturing method, when the method further comprises the steps of adding 5-30 wt % of binder to the oxide powder; and granulating the oxide powder, the molding density of the granulated particles can be further increased.
As the example of the binder, water, ethyl alcohol, carboxyl methyl cellulose, hydroxy propyl cellulose, polyvinyl alcohol, polyvinyl butyral, polyethylene glycol, poly acrylic acid ester or the like can be suitably used.
When the addition amount of the binder to the oxide material powder is excessively small to be less than 5 wt %, the effect of increasing the density by bonding the powders to each other with a high strength becomes insufficient. On the other hand, when the addition amount is excessively large so as to exceed 30 wt %, the ratio of the oxide powder in the molded body is excessively lowered thereby to decrease the molding density. Therefore, the addition amount of the binder is specified to a range of 5-30 wt %.
The added binder is removed by conducting a degreasing treatment to the granulated body or the molded body. Then, the degreased molded body or the like is sintered thereby to prepare the cold accumulating material of the present invention.
Other than the aforementioned method comprising the steps of spherically granulating the material powder through the rolling granulation method to form the granulated particles; and sintering the granulated particles, there can be also adopted the following method in which the material powder is spheroidized by utilizing a heat plasma.
That is, there can be also adopted a method of manufacturing cold accumulating material comprising the steps of: granulating the oxide particles having a predetermined composition; melting the granulated oxide particles by being passed through heat-plasma to form a molten liquid; and solidifying the molten liquid in a state where the molten liquid is spheroidized by the action of surface tension of the molten liquid thereby to prepare a cold accumulating material composed of a number of magnetic particles.
The method of granulating the aforementioned oxide particles is not particularly limited. For example, various granulating methods such as rolling-type, extruding-type, atomizing-type (spraying-type) can be used. As the material powder, a powder having an average grain size of 0.3-30 xcexcm is suitable. The more preferable average grain size of the material powder is 0.5-20 xcexcm. Further, a range 1-10 xcexcm is furthermore preferable.
In this regard, xe2x80x9cheat plasmaxe2x80x9d means a state of a gas being discharged at a high temperature. The heat plasma can be generated by the discharge of the gas by the action of direct current or high frequency electromagnetic wave having a frequency of several MHz to several GHz.
FIG. 3 shows a structure of a heat plasma apparatus. This heat plasma apparatus 80 comprises: a reaction vessel 81; a high-frequency oscillator 82; a coil 83; an outer cylinder for enclosing a plasma generating portion 86; a powder supplying port 86 which is opened so as to oppose to a plasma flame 85 generated at a top portion of the reaction vessel 81; a carrier gas supplying bomb 88 for carrying a powder stored in a powder supplying container 87 to the reaction vessel 81; a gas source 89 for generating the plasma; a cyclone 90 for separating the generated particles; and a cooling-gas source 91 for cooling the reaction vessel 81.
In the above heat plasma apparatus 80, the electromagnetic wave oscillated from the high-frequency oscillator 82 is amplified by the coil 83, while the gas supplied from the gas source 89 causes a discharge, so that a plasma flame 85 having a high temperature is generated at the top portion of the reaction vessel 81. The gas temperature of the flame portion 85 attains to several thousands xc2x0 C. to about 10,000xc2x0 C.
When the oxide particles together with the carrier gas supplied from the powder-supplying container 87 are thrown into the plasma flame 85 being under a state of high temperature, an entire particle or a part of the particle including a surface portion thereof is molten. The molten material powder is spheroidized by the action of the surface tension thereof. Then, the molten material is rapidly cooled and solidified by the cooling gas supplied from the cooling gas source 91 to form magnetic particles. Thus spherically formed magnetic particles are separated by the cyclone 90 and then recovered. As mentioned above, at least part of the material is molten, and then rapidly cooled and solidified in a spheroidized state, so that there can be obtained the magnetic particles having no crack on the surfaces thereof, a smooth surface resulting in a small surface roughness, and having no pore or void within the particles.
However, the magnetic particles spheroidized through the heat plasma method are manufactured by rapidly cooling the molten material in a state of high temperature of several thousands xc2x0 C., a particle texture or crystal structures such as perovskite structure capable of exhibiting a good specific heat characteristic cannot be obtained under some material compositions or treating conditions or the like, so that there is a case where a complicated structure including amorphous phase and crystal phase different from the aimed phase are disadvantageously formed. Accordingly, an inherent specific heat characteristic for the magnetic particles cannot be obtained, so that there has been posed a problem of lowering the refrigerating capacity of a refrigerator.
To cope with the above problem, as one effective embodiment of the method of manufacturing the cold accumulating material according to the present invention, it is preferable that the method further comprises the step of conducting a heat treatment at a temperature of 500xc2x0 C. or more with respect to the magnetic particles spherically formed by being passed through the heat plasma.
When the magnetic particles having crystal phases different from the aimed phase or having non-equilibrium phases such as amorphous phase or the like generated by being rapidly cooled from a state of high temperature in the heat plasma is subjected to the heat treatment at a temperature of 500xc2x0 C. or more, it becomes possible to re-synthesize the crystal phases different from the aimed phase and the non-equilibrium phases into the aimed crystal phases such as perovskite phase or the like.
When the temperature of the aforementioned heat treatment is less than of 500xc2x0 C., the effect of the re-synthesis of the crystal phases is not sufficient. In this regard, although a high temperature is preferable for the heat treatment, when the temperature exceeds a temperature 50xc2x0 C. lower than a melting point of the magnetic particles, a part of the magnetic particles starts to melt, thus being not preferable.
In view of the treating time and the limited specification of the heat-treating furnace, the temperature for the heat treatment is preferably set to 1800xc2x0 C. or less. Further, the more preferable temperature range for the heat treatment is 1000-1750xc2x0 C., and the best temperature range for the heat treatment is 1200-1700xc2x0 C. Although a time for the heat treatment is not particularly limited, the time is set to a range of 10 minutes to 50 hours. In addition, air or oxygen is preferable as an atmosphere for the heat treatment.
The cold accumulation type refrigerator of the present invention is constituted by being assembled with a regenerator (cold accumulating unit) into which the aforementioned cold accumulating material is packed as at least part of a cold accumulating material. The refrigerator of the present invention can be also constructed in such a manner that the cold accumulating material of this invention is packed into a regenerator for a predetermined cooling stage of the refrigerator while other cold accumulating materials such as Pb, HoCu2, and Er3Ni or the like having a specific heat characteristics corresponding to a required temperature distribution are packed into the same or another regenerator in a combined manner.
According to the cold accumulating material constructed as above, the equivalent circle diameter of the crystal grain, density, amount of additives (composition), amount of impurities, amount of defects such as crack and pore or void or the like are specified to a predetermined range, so that there can be provided the cold accumulating material having a high mechanical strength, a high thermal conductivity and an excellence in a thermal shock resistance, and having no fear of being finely pulverized.
Therefore, even if the cold accumulating material is used as a cold accumulating material for the refrigerators such as Starling refrigerator and a pulse-tube type refrigerator to be operated with a high speed, a pressure loss at the regenerator is small and there can be provided a cold accumulating material capable of exhibiting a stable refrigerating performance for a long time of period.
Further, when the above cold accumulating material is used as at least part of a cold accumulating material for the refrigerator, there can be provided a refrigerator having a high refrigerating performance at low temperature range, and capable of maintaining a stable refrigerating performance for a long time.
Furthermore, in an MRI apparatus, a cryopump, a super-conducting magnet for magnetic floating train, and a in-magnetic field single crystal pull-up apparatus (magnetic field applied Czochralski), since, in all of them, performance of the refrigerator dominates the performance of each apparatus, an MRI apparatus, a cryopump, a super-conducting magnet for magnetic floating train, and an in-magnetic field single crystal pull-up apparatus in which the above described refrigerators are assembled therein can exhibit excellent performances for a long term.